Nonrotating piston trimmer capacitor

ABSTRACT

A piston trimmer capacitor with a nonrotating piston comprises a tube of a dielectric material (glass) externally carrying a fixed cylindrical capacitor plate. The piston is a hollow metal cup within the tube, one end of which is threadably engaged to a longitudinal lead screw to permit positioning of the piston with respect to the external plate. Two openings with flat surfaces are formed in the end of the piston. These engage flats on two guide rails of special construction rigidly affixed to a bushing in one end of the tube. The engagement of the two sets of flats effectively prevent rotation of the piston without binding or interference with the axial motion of the piston in the tube. It also provides an excellent electrical connection between bushing and piston with a minimum of resistance and inductance. The bushing with its guide rails is preferably machined from a single piece of cylindrical stock. The outside diameter of the rails is just slightly less than the inside diameter of the piston. The flats are machined and the cylindrical stock is slotted on the diameter to accommodate the lead screw and bridge at the end of the cap, which leaves two solid arcuate legs dependent from the side of each rail, which legs effectively increase the rigidity of the guide rails and their resistance to twisting under torques.

United States Patent [72] Inventors Martin J. Blickstein Caldwell; Martin A. Mittler, Parsippany, both of, NJ. 121] App]. No. 869,412 [22] Filed July 9,1969 [45] Patented June 8, 1971 [73] Assignee Voltronics Corporation Hanover, NJ. Continuation-impart of application Ser. No. 603,262, Dec. 20, 1966, now abandoned.

[54] NONROTATING PlSTON TRIMMER CAPACITOR 2 Claims, 2 Drawing Figs.

[52] 11.8. C1 317/249 [51] lnt.Cl [101g 5/14 [50] Field of Search 317/249; 1/249 (T) [56] References Cited UN lTED STATES PATENTS 3,263,140 7/1966 Lafer 3l7/249(T) 3,286,140 11/1966 Camazza.. 3l7/249(T) 3,360,697 12/1967 Seiden 3 l7/249(T) 3,426,256 2/1969 Vinz 3 l 7/249(T) Hi --24 l9 l7 Primary Examiner-E. A. Goldberg Atrorney Burgess, Dinklage and Sprung ABSTRACT: A piston trimmer capacitor with a nonrotating piston comprises a tube of a dielectric material (glass) externally carrying a fixed cylindrical capacitor plate. The piston is a hollow metal cup within the tube, one end of which is threadably engaged to a longitudinal lead screw to permit positioning of the piston with respect to the external plate. Two openings with flat surfaces are formed in the end of the piston. These engage flats on two guide rails of special construction rigidly affixed to a bushing in one end of the tube. The engagement of the two sets of flats effectively prevent rotation of the piston without binding or interference with the axial motion of the piston in the tube. It also provides an excellent electrical connection between bushing and piston with a minimum of resistance and inductance. The bushing with its guide rails is preferably machined from a single piece of cylindrical stock. The outside diameter of the rails is just slightly less than the inside diameter of the piston. The flats are machined and the cylindrical stock is slotted on the diameter to accommodate the lead screw and bridge at the end of the cap, which leaves two solid arcuate legs dependent from the side of each rail, which legs effectively increase the rigidity of the guide rails and their resistance to twisting under torques.

NONROTATING PISTON TRIMMER CAPACITOR RELATED APPLICATION The application is a Continuation-in-Part of Piston Trimmer Capacitor," Ser. No. 603,262, filed Dec. 20, 1966 now abandoned.

PRIOR ART A related type of piston trimmer capacitor is shown in US. Pat. No. 3,263,140 (Lafer). This design has been tested, however, and it has been found that the guide rails bend or twist easily and that the capacitor will not pass MlL-Cl4409.

THIS INVENTION The piston trimmer capacitor of this invention is characterized by having a nonrotating piston as the adjustable plate of the capacitor. The piston is contained within a tube of a dielectric which supports a fixed tubular plate thereabout.

The axial displacement of the piston is accomplished by a lead screw which extends longitudinally through a support bushing onto which one end of the tubular member is inserted.- The lead screw is received within the support bushing for rotation relative thereto, extends into the interior of the tubular member, and is connected to a transversely extending bridge portion of the piston for rotation relative thereto.

Rotation .of the piston is prevented by a pair of guide rails oppositely disposed about the lead screw and extending from the support bushing longitudinally within the tubular member, each guide rail passing through a corresponding internal slot provided in the, piston. These slots are oppositely disposed about the bridge portion of the piston, which expediently is a hollow cylindrical piston having an end cover portion apertured to define such guide slots and bridge portion. Each guide rail has a similarly flattened guide surface portion facing transversely outward with respect to the axial path of piston movement, and disposed for sliding contact engagement with corresponding flattened contour portions of respective piston guide slots.

The guide rails have inside surface portions which face the piston bridge, but are spaced therefrom by a clearance distance so that neither guide rail touches the bridge at any axial position of the piston. This clearance is essential for if the guide rail touches the bridge, this interference will cause the mechanism to jam. The tendency of the piston to rotate with the lead screw is blocked by the mechanical interfering action between the flattened slots and guide rails alone, and consequently no bridge-to-guide rail contact is needed or utilized to block piston rotation. To assure that the mechanical interference between the guide rails and piston slots will be of a type which only prevents piston rotation, and allows free axial movement of the piston along the effective length of the guide rails, the flats on the outside of the guide rails are machined parallel to the axis of piston movement.

By making the guide rails so as to clear the piston bridge, and utilizing outside flat guide surfaces on the guide rails to block piston rotation, several advantages are realized by the invention. For example, to accommodate the connection of the lead screw to the piston, within a minimum diameter piston size, the central portion of the bridge is ordinarily circular in shape and projects beyond the width of the legs of the bridge. To machine a guide rail or a pair of opposite guide rails which will reach around the curved central portions of the bridge to engage the legs thereof is more complicated and expensive than to machine parallel flats on the outsides of the guide rails and to create matching flat contoured slots in the piston end cover. Also, a greater reduction in piston rotary motion backlash can be achieved with outside flat guide rail surfaces.

Actually, a single guide rail and piston slot combination will suffice to prevent piston rotation, but a pair of guide rails and piston slots are preferably provided for better support and to minimize piston rotary backlash. Three or more rails can be used, if desired.

The invention provides a trimmer capacitor construction affording a low-loss electrical conduction path to the moveable capacitor plate defined by the piston, which is preferably made of metal. Prior art piston trimmer capacitors which rely entirely upon conduction from the piston through the lead screw sufier relatively higher losses, and thus have a lower 0- factor than the capacitor of the invention because of the inherently higher resistance across the rotary joint lead screwto-piston connection. By making the support bushing and guide rails of metal, and preferably of monolithic construction, additional, low-loss, high conductivity paths from the piston to the support bushing are created through each guide rail, and whatever piston-to-support bushing conductivity path exists through the lead screw is not discarded, but rather added in parallel to those provided through the guide rails. The support bushing thus becomes a terminal to accommodate the electrical connection of the moveable capacitor plate defined by the piston to an external circuit, and for such purpose the support bushing can have a threaded shank to allow convenient installation of a solder lug or other connection aid, as well as for mounting purposes.

The capacitors are often used at very high frequencies where the current runs on the outer surface. It is particularly desirable, therefore, to have a good electrical contact between the guide rails and the piston as this provides the lowest resistance with the least amount of inductance, which is quite important in a high frequency capacitor. Inductance is minimized in the present design by keeping the current path as small as possible.

Since the support bushing is to be utilized as a piston plate terminal, the fixed capacitor plate defined by the tubular member must necessarily be insulated from the metallic support bushing. This is easily accomplished by making the tubular member from a tube of insulating material, such as glass or plastic, which tube can constitute the principal dielectric of the capacitor, and by providing a metallic layer disposed in contact with the exterior surface of the insulating tube for support thereby. This metallic layer constitutes the fixed capacitor plate and accordingly is spaced away from the end of the tube which is inserted onto the support bushing as required to prevent short circuiting. A pigtail lead can be wrapped around the metallic layer and soldered thereto to facilitate external circuit connections.

To assure a better piston-to-guide rail conductivity, the guide rails can be spread apart slightly so that they resiliently engage against the piston slots without any significant frictional resistance to the axial movement of the piston.

THE DRAWINGS FIG. 1 is a longitudinal view, partly in section, of a piston trimmer capacitor according to a preferred embodiment of the invention wherein capacitance adjustment can be accomplished by rotating the lead screw in a fixed axial position; and

FIG. 2 is an end view of the piston trimmer capacitor shown in FIG. 1, as soon looking toward the support bushing thereof.

DESCRIPTION AND EXAMPLE Referring now to FIGS. 1 and 2, the piston trimmer capacitor 10 of the invention utilizes a tubular member 11 for a fixed capacitor plate and a piston member 12 for a moveable capacitor plate. One end 13 of the tubular member 11 is inserted onto a support bushing 14 which can be threaded as at 15 to accommodate mounting of the capacitor 10 to a circuit board, chassis, etc. (not shown).

The piston 12 has an end cover portion 16 which is apertured to define a pair of guide slots 17 oppositely disposed about a transversely extending bridge portion 18.

A rotatable lead screw 19 is provided for adjusting the axial position of the piston 12 within tubular member 11. Tubular member 11 is made up of a tube 20 of electrical insulation material, which constitutes the principal dielectric of capacitor l0, and a metallic layer 21, preferably cylindrical and constituting the fixed plate of capacitor 10.

Piston 12 is made of metal, and is preferably cylindrical in shape, and of a length comparable to that of the fixed plate layer 21. The tubular member 11, layer 21 and lead screw 19 are preferably of such a length in relation to the length of piston 12 as to allow said piston 12 to be positioned almost entirely within the axial length zone subtended by layer 21 for maximum capacitance, and to be completely withdrawn from such axial zone for. minimum capacitance. As a general rule, the piston 12 and layer 21 are approximately equal in length, and equal to about one-half of the tubular member 11 length.

Lead screw 19 extends longitudinally through support bushing 14 and is rotatable relative thereto, and also is connected to piston bridge portion 18 for rotation relative thereto when adjusting the axial position of piston 12 within tubular member 11 to correspondingly vary the effective capacitance of capacitor 10.

In the preferred embodiment of the invention represented .by HG. 1, the lead screw 19 does not undergo any axial displacement when tumed in either direction to shift piston 12. Hence, no allowance need be made foroutward movement of the slotted adjustment end 22 of lead screw 19, a feature which saves space in crowded installations. Lead screw l9is provided with a flange 23 which abuts against a shoulder 24 and retaining insert 25 provided withinsupport bushing 14, said flange 23, shoulder 24 and insert 25 serving to constrain lead screw 19 against axial movement relative to support bushing 14 and yet permit free lead screw 19 rotation relative .thereto. Piston bridge 18 is provided with a threaded bore 26 and lead screw 19 is connected to piston 12 in threaded engagement through its bridge bore 26.

A pair of guide rails 27 extending longitudinally from support bushing 14 .within tubular member 11 pass through corresponding guide slots 17 in sliding contact engagement therewith to prevent rotation of piston 12 when lead screw 19 is turned. These guide rails 27 have similar flattened guide surfaces 28 facing transversely outward with respect to the axial path of piston 12 movement, and are arranged for sliding contact with corresponding flattened contour portions 29 of respective piston guide slots 17, as better seen from FIG. 2.

The inside surfaces 30 of guide rails 27 are arranged to clear the bridge 18, and thus, the nonrotating axial movement of piston 12 under the influence of lead screw 19 is accomplished entirely by the antirotation mechanical interference between guide surfaces 28 and slot flats 29. This is advantageous from the viewpoint of simplicity and economy of construction, because the dimensional tolerances on the location of the inside surface 30 becomes noncritical, except for clearing the bridge 18. It is expedient to machine flatguide surfaces 28 and slot flats 29 than to machine the inside contour of guide rails 27 to reach around the bulging central portion 31 of bridge 18 and engage against the leg portions 32 thereof.

The guide rails have greater rigidity imparted to them by virtue of the fact that they have, in cross section, dependent legs, preferably ones that conform somewhat to the arcuature shape of the piston walls. These legs increase the effective thickness, and thus rigidity of the rails. If the guide rails were only rectangular in cross section, they could not be made rigid enough from a practical standpoint, within the limited space provided in most commercial and military capacitors. For example, for a capacitor having a piston with an inside diameter of 170 mils, the maximum width rail, rectangular in cross section, that could be accommodated is about 100 mils, at a rail thickness of 8 mils. The maximum rail thickness is about 20 mils, at a width of 60 mils. A rail could not be 100 mils wide and 20 mils thick because there would not be enough room for it. A rectangular rail structure could not meet, for example, a turning torque test (See MlL-C 14409) after the piston has been moved to the stops of greater than inch-ounces, regardless of the material of construction.

in this invention, the rails 27 are preferably machined from cylindrical stock that can have an outside diameter just about equal the inside diameter of the piston; about to 250 mils, e.g. about 200 mils; which leaves sufficient clearance for the bridge and screw with its retaining means or nut. The flat outside surface 28 of about 50 to mils, e.g. 85 mils, is cut on a chord of the circular cross section, which leaves a minimum wall thickness from the flat surface 28 to the inside of the cylinder of l0 mils. A slot is cut through the cylinder on the diameter, as shown, to accommodate the piston bridge, such that the total thickness of the rail from surface 28 to the bottom of the legs is about 30 to 60 mils, i.e., l 0 to 40 mils thicker than the thinnest wall thickness between the guide surface and the inner diameter of the cylindrical stock. Rails made in this manner as from brass, will readily pass aturning torque test at the stops of greater than 15 inch-ounces.

Guide rails 27, support bushing 14 and piston 12 are-made of metal so as to provide an electrically conductive path extending from the moveable capacitor plate defined by piston 12 to the support bushing 14. Preferably, guide rails 27 and support bushing 14 are integrally constructed to eliminate any resistive losses due to connection joints. Lead screw 19 is expediently made of metal, and thus provides an additional conductive path from lead screw 19 to piston 12, although such path offers a somewhat higher resistance than that of either conductive path through each guide rail 27. In the conductive path through lead screw 19, there are two rotary joints in series, i.e., the threaded rotary connection between lead screw 19 and piston bridge 18, and the free rotary connection between lead screw 19 and support bushing 14, whereas in each guide rail 27 conductive path, there is only one moveable connection, i.e., the sliding contact between guide surface 28 and slot flat 29.

' To improve piston 12 to guide rail 27 electrical conductivity, the guide rails 27 can be spread apart slightly to resiliently engage against slot flats 29, withoutany significant increased frictional resistance to piston 12 movement.

From FIG. 2 it can readily be noted as to how the slot flats 29 mechanically interfere with guide surfaces 28 on guide rails 27 to prevent piston 12 rotation by any reaction torques transmitted by lead screw 19 rotation. The flattened guide surface 28 and slot contours 29 are merely presented as one illustrative example of how piston 12 rotation can be prevented by contact with the outside surface of guide rails 27.

Only one guide rail 27 and associated piston slot 17 will suffice to prevent piston 12 rotation. However, a pair of guide rails 27 and piston slots 17 are preferably used as exemplified in FIGS. 1-2 to minimize'any piston 12 rotary movement backlash as might result from guide rail 27 and slot 17 fitting tolerances, were only one guide rail 27 and slot 17 to be provided. Such backlash can be also minimized by slightly spreading apart guide rails 27, as done to improve piston 12-to-support bushing 14 conductivity.

What we claim is:

1. In a piston trimmer capacitor having a tubular dielectric member externally carrying a fixed capacitor plate, a support bushing at the end of said tubular dielectric member, a hollow piston within said tubular dielectric member defining a moveable nonrotating capacitor plate disposed for axial movement relative to said fixed capacitor plate, said piston having a transverse bridge portion, and a lead screw extending longitudinallythrough said support bushing and connected to said bridge portion for rotation relative thereto to selectively adjust the relative position of said piston with respect to said fixed capacitor plate; the improvement comprising guide slots within a transverse wall of said piston having outer portions of flattened contour on opposite sides and two opposed substantially rigid guide rails extending from said support bushing longitudinally therethrough, said rails and support bushing being a single monolithic piece machined from a single piece of cylindrical stock, the outside diameter of which is slightly less than the inside diameter of said piston, each of said rails having a peripheral flattened outer guide surface mating with said thereby contributing to the rigidity of said guide rail.

2. The capacitor of claim 1 wherein said bridge portion and said transverse wall comprise an end wall of said piston, and the other end of said piston being open. 

1. In a piston trimmer capacitor having a tubular dielectric member externally carrying a fixed capacitor plate, a support bushing at the end of said tubular dielectric member, a hollow piston within said tubular dielectric member defining a moveable nonrotating capacitor plate disposed for axial movement relative to said fixed capacitor plate, said piston having a transverse bridge portion, and a lead screw extending longitudinally through said support bushing and connected to said bridge portion for rotation relative thereto to selectively adjust the relative position of said piston with respect to said fixed capacitor plate; the improvement comprising guide slots within a transverse wall of said piston having outer portions of flattened contour on opposite sides and two opposed substantially rigid guide rails extending from said support bushing longitudinally therethrough, said rails and support bushing being a single monolithic piece machined from a single piece of cylindrical stock, the outside diameter of which is slightly less than the inside diameter of said piston, each of said rails having a peripheral flattened outer guide surface mating with said flattened contour, and an inner surface facing said lead screw that clears said bridge portion, and said rails having dependent leg portions on either side which are arcuate sections of said cylindrical stock and extend substantially beyond the thinnest section between said outer guide surface and inner surface thereby contributing to the rigidity of said guide rail.
 2. The capacitor of claim 1 wherein said bridge portion and said transverse wall comprise an end wall of said piston, and the other end of said piston being open. 